A black hole is a volume from which photons, or any matter, can not escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

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Is it possible to have a singularity with zero mass?

A singularity, by the definition I know, is a point in space with infinite of a property such as density. Density is Mass/Volume. Since the volume of a singularity is 0, then the density will thus ...
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Hawking radiation and black hole entropy

Is black hole entropy, computed by means of quantum field theory on curved spacetime, the entropy of matter degrees of freedom i.e. non-gravitational dofs? What is one actually counting?
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How can we detect a black hole? [duplicate]

If black holes are phenomena of very high density (gravitational singularities) which don't emit radiation how can we detect them so far away from us where so much other radiation can hide the black ...
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Spaceship split near event horizon

Lets say there's two astronauts, Alice and Bob, going on a space trip to a super-massive black hole. So large that they wouldn't notice any significant spaghettification forces at the event horizon. ...
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396 views

Firewall's grandfather paradox

See What are cosmological "firewalls"?. Alice is in freefall in her spacecraft just above the horizon of a gigantic black hole. She measures whether or not the near modes of the horizon ...
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162 views

Why doesn't the firewall argument also apply to far away ingoing modes?

Gidom Mera's answer at http://physics.stackexchange.com/a/45511 is illuminating, but on closer analysis, it brings up further puzzles. Backscattering works in both directions. Let's see what we get ...
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How would you detect Hawking radiation?

Hawking theorized that a black hole must radiate and therefore lose mass (Hawking radiation). According to classical relativity though, nothing can escape a black hole, the hawking radiation would ...
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201 views

Relativistic Computation?

Is it possible to employ relativity to develop computational technology? Here is a really basic example: Build a Computer and Feed it the Problem (say the problem is projected to take 10 years to ...
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Singularities in Schwarzchild space-time

Can anyone explain when a co-ordinate and geometric singularity arise in Schwarzschild space-time with the element $$ ...
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799 views

Analog Hawking radiation

I am confused by most discussions of analog Hawking radiation in fluids (see, for example, the recent experimental result of Weinfurtner et al. Phys. Rev. Lett. 106, 021302 (2011), ...
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195 views

What happens to things when things get crushed in a blackhole [duplicate]

When a black hole destroys things until they are smaller than molecules, where does it go and what happens when it clogs up?
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214 views

Does the curvature of space-time cause objects to look smaller than they really are?

What's the difference between looking at a star from a black hole and looking at it from empty space? My guess is that the curvature of space-time distorts the wavelength of light thus changing the ...
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Why does the Schwarzschild radius become excessively large after a certain point?

Here's something that I've found difficult to wrap my head around. The relationship between the Schwarzschild radius and mass is linear. It's generally known that if you take an object in the universe ...
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Where 2 comes from in formula for Schwarzschild radius?

In general theory of relativity I've seen several times this factor: $$(1-\frac{2GM}{rc^2}),$$ e.g. in the Schwarzschild metric for a black hole, but I still don't know in this factor where 2 comes ...
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How much of a star falls into a black hole?

http://blogs.discovermagazine.com/badastronomy/2011/04/05/astronomers-may-have-witnessed-a-star-torn-apart-by-a-black-hole/ A lot of the star in the disc, a lot of the star in the jets, precisely how ...
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Black Hole Photon Sphere

The photon sphere is a spherical region in space where photons are forced to travel in an orbit at $r = \frac{3GM}{c^{2}}$. Is it possible to detect these spheres? What happens if I fall through ...
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General definition of an event horizon?

Horizons are in general observer-dependent. For example, in Minkowski space, an observer who experiences constant proper acceleration has a horizon. Black hole horizons are usually defined as ...
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How to get Planck length

I know that what Planck length equals to. The first question is, how do you get the formula $$\ell_P~=~\sqrt\frac{\hbar G}{c^3}$$ that describes the Planck length? The second question is, will any ...
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What is mathematical definition of a strong gravity?

Mathematical definition of a weak gravity is simple $g=\frac{GM}{r^2}$ but what is mathematical definition of a strong gravity? (blackhole-like or close to a blackhole-like object)
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Why are black holes special?

A black hole is where it's mass is great enough that light can't escape at a radius above the surface of the mass? I've been told that strange things happen inside the event horizon such as ...
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Overcharging a black hole

Hubeny's 1998 paper got a lot of people interested in determining whether cosmic censorship can be violated by dropping too much charge onto a black hole. It suggested that you might be able to get a ...
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How small the earth should be for it to become a black hole

How small would the earth have to be squashed so that it would become a black hole?
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808 views

What would happen to the Moon if Earth is turned into a black hole?

Assume that all of sudden the Earth is turned into a black hole. And the moon revolves around the Earth (before turning into a black hole). What would happen to the Moon after earth changes to black ...
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Why is a black hole black?

In general relativity (ignoring Hawking radiation), why is a black hole black? Why nothing, not even light, can escape from inside a black hole? To make the question simpler, say, why is a ...
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Magnetic field-pulsed microwave transmission line

Here's the reference: The researchers showed that a magnetic field-pulsed microwave transmission line containing an array of superconducting quantum interference devices, or SQUIDs, not only ...
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Time inside a Black hole

If time stops inside a black hole, due to gravitational time dilation, how can it's life end after a very long time? If time doesn't pass inside a black hole, then an event to occur inside a black ...
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1answer
243 views

The paradoxical nature of Hawking radiation [duplicate]

The definition of a classical black hole is when even electromagnetic radiation can not escape from it. Why then can Hawking radiation be emitted from semi-classical black holes? What is difference ...
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1answer
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What is baryon loading in the context of gamma ray bursts (GRBs)?

I've read that with short hard gamma ray bursts (shGRBs) associated with the coalescence of NS-NS and NS-BH binaries are expected to be beamed along the axis of the orbital angular momentum (i.e. ...
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Is black hole bright at center?

As we know that light photon cannot escape the gravity of black hole so I was thinking that if that is the surface of the black hole would be bright as all the photons would be there only. Am I right ...
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Anti-matter black hole and time

I have recently read some hard science-fiction story based on an assumption that if time stops (from external observer's perspective) on the event horizon of black hole, then in an anti-matter black ...
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Do semiclassical GR and charge quantisation imply magnetic monopoles?

Assuming charge quantisation and semiclassical gravity, would the absence of magnetically charged black holes lead to a violation of locality, or some other inconsistency? If so, how? (I am not ...
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black hole no-hair theorems vs. entropy and surface area

I was revisiting some old popular science books a while ago and two statements struck me as incompatible. No-hair theorems: a black hole is fully-described by just a few numbers (mass, spin etc) ...
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No hair theorem for black holes and the baryon number

The no hair theorem says that a black hole can be characterized by a small number of parameters that are visible from distance - mass, angular momentum and electric charge. For me it is puzzling why ...
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4answers
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If an anti-matter singularity and a normal matter singularity, of equal masses, collided would we (outside the event horizon) see an explosion? [duplicate]

If an anti-matter singularity and a normal matter singularity, of equal masses, collided would we (outside the event horizon) see an explosion?
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Can you enter a timelike hypersurface?

As I understand it, a timelike hypersurface is one that has only spacelike normal vectors. But does this not imply that a the geodesic of a particle crossing it must be spacelike at that point? But ...
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Diving into a charged (Reissner-Nordstrom) Black hole

Apparently there are two event horizons in this type of black hole, where the second one is known as the Cauchy horizon. According to Carroll, if you go into the first one, you will fall until you ...
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Reconstruction of the initial state from Hawking radiation?

I hear that unitary evolution and information conservation must imply that information about information content that defines the initial state of matter used to create a black hole can be inferred ...
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677 views

The Uncertainty Principle and Black Holes

What are the consequences of applying the uncertainty principle to black holes? Does the uncertainty principle need to be modified in the context of a black hole and if so what are the implications ...
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Future light cones inside black hole

In Caroll's Spacetime and Geometry, page 227, he says that from the Schwarzschild metric, you can see than from inside a black hole future events all lead to the singularity. He says you can see this ...
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Are quarks the limits? The end of the fundamental science. For collisions on more higher energies will lead to black holes after all?

Are quarks the limit and the Plank scale is believed to be the limit of distance when the very concept of space and length cease to exist (10^-19) Any attempt to research the existence of shorter ...
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What happens when one black hole eats another black hole? [duplicate]

Everyone seems to know about black holes. They have an absolutely massive mass, and if you go into them, time stops from an outside perspective. When a planet gets too close to a black hole, it is ...
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168 views

Is singularity at the exact centre of a black hole?

I've read that all paths inside the EH lead to singularity. ALL paths. Even the ones pointing away from it, right? Because there's NO pointing away from singularity, since ALL paths point to it. So ...
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Has the black hole information loss paradox been settled?

This question was triggered by a comment of Peter Shor's (he is a skeptic, it seems.) I thought that the holographic principle and AdS/CFT dealt with that, and was enough for Hawking to give John ...
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Falling into a black hole emitter vs observer

Let's say we are working with the Schwarzschild metric and we have an emitter of light falling into a Schwarzschild black hole. Suppose we define the quantity $$u=t- v$$ where $$dv/dr= ...
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Spacelike slicing of Schwarzschild geometry

I am having trouble understanding how to obtain a spacelike slicing of the Schwarchild black hole. I understand there is not a globally well defined timelike killing vector, so we can define t=cte ...
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2answers
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Which way does a black hole spin?

As far as I understand, and from what I have been shown in renderings of black holes, they spin (like water going down a drain). My question is, firstly, does the matter being pulled into a black ...
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Relaxation time for deviations from spherical shape of a black hole's event horizon (and waves)

A different question about truly spherical objects in nature (Do spheres exist in nature?) made me think of a lecture I had been at where, as I recall, it was mentioned that the most perfectly ...
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1answer
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Is it possible that a black hole sucks energy that is the origin of another universes big bang?

The universe expanded from nothing, right? And black holes may be a "gate" to parallel universes, is it possible that stars that is being sucked in by black holes in our universe may be the origin of ...
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about the 1D singularity of black hole

I saw some responses here saying that the singularity into the black hole is one dimension object so my question is : is it possible that the singularity is simply a merger of the 4 dimensions of the ...
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How small are the smallest black holes?

How little mass can a black hole contain and still be a "stable" black hole? What would the diameter be, in terms of the event horizon?